In lightwave communication systems which utilize optical fiber as a transmission medium, polarization-mode dispersion (PMD) often presents a significant obstacle to achieving higher data rates and/or longer repeaterless transmission distances. PMD refers generally to variations in the delay time of an optical signal through the medium as a function of .signal polarization, which can cause randomly varying pulse width distortion in the received signal. PMD arises in optical fiber because a fiber core is usually not perfectly symmetric, and therefore the propagation speed of a signal at one polarization, such as an s-polarization, may be different than the propagation at another polarization, such as a p-polarization. In a single-mode fiber, at each signal wavelength there generally exists a pair of orthogonal input polarization states, referred to as the principle states of polarization (PSPs) in the fiber, for which the corresponding states of polarization at the output of the fiber are orthogonal and independent of wavelength to first order. See, for example, C. D. Poole and R. E. Wagner, "Phenomenological Approach to Polarization Dispersion in Long Single-Mode Fibres," Electronics Letters, Vol. 22, pp. 1029-1030, September, 1986, which is incorporated by reference herein. Ignoring wideband frequency-dependent effects such as fiber chromatic dispersion, and to a first-order approximation, an optical signal transmitted through the fiber in either fiber PSP is generally undistorted at the receiver but has a different time delay depending upon the PSP in which it was transmitted. An optical signal with an arbitrary state of polarization (SOP) can be expressed as a sum of signals in each fiber PSP, and when received after transmission through the fiber the signal can therefore be characterized as a combination of two orthogonally-polarized signals with different time delays. Because the fiber PSPs and corresponding time delays typically vary as a function of, for example, temperature and changes in position or other movement of the fiber, the received signal exhibits time-varying distortion. The fact that PMD-induced distortion manifests itself in single-mode fiber is of particular concern because much of the existing or "embedded" optical fiber infrastructure throughout the world utilizes this type of fiber.
A known technique for limiting PMD-induced distortion in a detected signal is described in U.S. Pat. No. 5,311,346, entitled "Fiber-Optic Transmission Polarization-Dependent Distortion Compensation," which is assigned to the assignee of the present invention and incorporated by reference herein. An embodiment of this exemplary technique utilizes a polarization controller to adjust the polarization of an optical signal at either the input or the output of a length of fiber. The signal polarization is adjusted such that a performance measure of received signal quality is optimized. Exemplary measures of signal quality which are utilized include bit-error-rate (BER), signal-to-noise ratio (SNR) and received signal eye opening. Although this system substantially reduces PMD-induced distortion, a significant number of additional components may be required to measure received signal quality in certain embodiments.
Another technique for reducing PMD-induced distortion, described in M. Yoshimura et al., "Polarization Mode Dispersion Equalization," Technical Digest, Fifth Optoelectronics Conference (OEC '94), Paper 14E-12, July, 1994, includes a PMD equalizing circuit which is driven by an error signal. The error signal is generated as the difference between a detected optical signal and a reference waveform derived from the detected signal. This technique uses an equalizing circuit which includes a number of different phase shifters and variable TE/TM converters to provide suitable optical signal adjustments in response to the error signal, and is therefore complicated, expensive to implement and overly sensitive to factors unrelated to PMD-induced distortion including, for example, chromatic dispersion. Other techniques are described in, for example, T. Okoshi et al., "New Polarization-Control Scheme for Optical Heterodyne Receiver Using Two Faraday Rotators," Electronics Letters, Vol. 21, No. 18, pp. 787-788, August, 1985, and F. Heismann et al., "Automatic Polarization Demultiplexer for Polarization-Multiplexed Transmission Systems," Electronics Letters, Vol. 29, No. 22, pp. 1965-1966, October, 1993. These techniques are generally directed to varying input signal polarization such that the detected signal power level in a particular polarization state is maximized. Because factors unrelated to PMD may alter detected signal power level, these and other prior art techniques may be unable to adequately compensate for PMD-induced distortion.
As is apparent from the above, a need exists for improved detection of optical signals received over single-mode optical fiber and other polarization-mode dispersive transmission media.